1. Field of the Invention
The present invention relates to an ignition laser for a laser ignition device of an internal combustion engine.
2. Description of Related Art
A so-called laser igniter is known from published international patent application document WO 2005/066488 A1. This laser igniter includes an ignition laser, which protrudes into the combustion chamber of an internal combustion engine. This ignition laser is connected via an optical fiber to a pump light source.
A so-called combustion chamber window is provided on one end of the ignition laser facing toward the combustion chamber, which is transmissive for the laser pulses generated in the ignition laser. The laser wavelength is between 500 nm and 1500 nm, preferably between 900 nm and 1100 nm. This combustion chamber window must be sealingly accommodated in a housing of the ignition laser. High requirements are placed on the seal between the combustion chamber window and the housing, because surface temperatures of greater than 600° C. may occur on the combustion chamber window during the operation in an internal combustion engine, for example. In addition, intermittent pressure loads of greater than 250 bar occur. If an ignition laser is used to ignite a gas turbine, lower pressures do prevail in the combustion chamber of the gas turbine; however, the surface of the combustion chamber window may reach temperatures of up to 1000° C.
In order for the laser igniter to operate without interference, the combustion chamber window must be transparent to the laser beams over the entire lifetime of the internal combustion engine. However, deposits may accumulate on the combustion chamber window during the operation, so that the required optical transparency of the combustion chamber window is lost. In order to prevent these deposits, the ignition laser is to be designed in such a way that the operating temperature on the surface of the combustion chamber window facing toward the combustion chamber is sufficiently high to burn off or oxidize deposits, above all of an organic nature.
However, the surface temperature of the combustion chamber window may not be so high that self-ignition of the fuel-air mixture may occur on the surface of the combustion chamber window or deposits, in particular oil ashes, may be formed on the combustion chamber window.
An ignition laser is known from published German patent application document DE 198 10 750 A1, in which the temperature of the combustion chamber window may be reduced by increasing the heat dissipation into the base of the ignition laser.
As a result, the surface temperature of the combustion chamber window in the entire operating range of the internal combustion engine must be kept above the burnoff temperature for organic coatings, which is approximately 400° C., and below a temperature of 600° C., from which a growth of crystalline phosphates from the oil ashes is known. This means that a temperature range between 420° C. and 550° C. is to be permanently achieved on the surface of the combustion chamber window.
Since this mentioned temperature range may not be maintained at least during the cold start of the internal combustion engine, rapid heating of the ignition laser or the combustion chamber window is desirable. Simultaneously, during steady operation of the internal combustion engine, the heat from the ignition laser must be sufficiently dissipated to avoid excess temperatures.